Protective system for highfrequency generators



Oct. 26, 1954 G. w. SCHROEDER PROTECTIVE SYSTEM FOR HIGH-FREQUENCY GENERATORS Filed April 18, 1951 George Schroeder ATTys.

Patented Oct. 26, 1954 PROTECTIVE SYSTEM FOR HIGH- FREQUENOY GENERATORS George W. Schroeder, Sandy Hook, Conn., assignor to General Electric Company, a. corporation .of New York Application April 18, 1951, Serial No. 221,630

12 Claims.

The present invention relates to protective systems for high-frequency generators and more particularly to such systems for protecting magnetron generators incorporated in ultra-high frequency heating apparatus.

Ultra-high frequency heating apparatus usually comprises a cavity adapted to receive semiconducting material that is to be heated, an. ultra-high frequency generator including a magnetron tube, and. an interposed transmission line that may be either of the coaxial conductor type or of the wave guide type. In such apparatus, it is highly desirable that the impedance of the load match the impedance of the generator, but this is not always possible since the impedance of the load depends upon the weight and character of the materialrthat is to be heated :in :the cavity, whereby some impedance mismatch is always present and causes :a reflection of ultra-high frequency power from the cavity back through the transmission line to the generator, theamount of reflected ultra-high frequency power being 'proportional to the amount of impedance mismatch and to the amount of incident power supplied from the generator to the cavity through the transmission line. In fact, should the apparatus be operated without any load in the cavity, the reflected ultra-high frequency power is substantially equal to the incident ultra-high frequency power. lhis reflected ultra-high frequency power produces a heating efiect upon the magnetron tube incorporated in the ultra-high frequency generator, which heating effect is proportional to the amount of reflected ultra-high frequency power. Thus in the event of a severe impedance mismatch, the reflected ultra-high frequency power is sufiiciently great to effect undue and damaging heating of the magnetron tube causing direct failure thereof by cracking of the glass seal that is disposed about the connection extending through the wall thereof to the plate arranged therein, notwithstanding the provision of the air blower and the water cooling coil that are normally operatively associated with the magnetron tube. Specifically it has been discovered that in the event the impedance mismatch is of such severity that the microwave voltage standing wave ratio exceeds a value of about 3.0, the magnetron tube will invariably fail.

Accordingly it is the general object of the present invention to provide an improved protective system for a high frequency generator so as to prevent damage thereto resulting from an impedance mismatch between the generator and the connected load.

Another object of the invention is to provide in a high frequency power supply system including a high frequency generator, an arrangement for selectively setting the amount of incident high frequency power outputfrom the generator in accordance with the amount of reflected high frequency power back to the generator.

Another object of the invention is to provide in a high frequency supply system, an improved circuit network for protecting the high frequency generator incorporated therein against damage as a result of reflected high frequency power back thereto and for selectively setting theincident high frequency power output from lithe generator in accordance with the amount of reflected high frequency power back thereto.

A further object of the invention is to provide in ultra-high frequency heating apparatus, an improved protective system of the character noted.

Further features of the invention pertain to the particular arrangement of the elements of the ultra-high frequency heating apparatus and the associated protectivecircuit network, whereby the above-outlined and additional opera-ting features thereof are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawing, in which the single figure is a diagrammatic illustration of ultra-high frequency heating apparatus and a power supply and protective circuit network therefor embodying the present invention.

Referring now to the drawing, the ultra-high frequency heating apparatus and the power supply and protective circuit network there illustrated, and embodying the features of the present invention, comprises a cooking oven [0 provided with a metal liner ll defining a cavity [2 arranged to receive semi-conducting material l3 that is to beheatecl, an ultra-high frequency generator arrangement including a magnetron tube V! that may be of General Electric Company type 2-1492, and an interposed transmission line I4. The transmission line I4 is of the coaxial conductor type including an outer grounded tubular conductor I5 and a centrally disposed insulatedrod-like conductor [6. The outer end of the transmission line H is bifurcated terminating in two arms embracing the oven 1 0. The extremities of the two outer conductors [5a and I5b of the two arms of the transmission line H are connected to the liner 1 I; while the extremities of the two inner conductors IBa and I617 of the two arms of the transmission line I4 respectively project through two associated openings formed in the liner I I and respectively terminate in two inwardly projecting antennas Ice and Mid disposed in the cavity I2. Also the two inner conductors I611 and IE1) of the two arms of the transmission line I l respectively carry two sleeve transformers I61: and I631 in order to insure that the impedance of the transmission line It matches the impedance of the magnetron tube VI. The magnetron tube VI comprises a filamentary cathode Ill and a plate I8, the plate 18 being electrically connected to the inner end of the inner conductor I5 of the transmission line M by the usual coupling loop I8a and to ground potential. The magnetron tube VI is capable of oscillating at an ultra-high frequency of the order of 1,000 megacycles, whereby centimeter electromagnetic standing waves are produced in the cavity I2 when the magnetron tube VI is operating.

An attenuator I9 is connected to the transmission line It and may take the form of a length of hollow conducting tubing constituting a section of wave guide with cross-sectional dimensions suificiently small so as not to propagate waves of the frequency in the transmission line I4. The attenuator I9 is secured to the outer conductor !5 of the transmission line I l in surrounding relation with respect to an opening formed in the outer conductor I5 so that a portion of the field in the transmission line I4 may extend into the attenuator I9. An element V9 in the form of a gaseous discharge tube is arranged within the attenuator i9 and positioned away from the transmission line it toward the outer end of the attenuator I9 a distance corresponding to the maximum field intensity to which it is desired to subject the element V9, it being understood that the intensity of the field within the attenuator I9 decreases with distance away from the transmission line it toward the outer end thereof. Specifically the gaseous discharge tube vs may take the form of a /4 watt neon glow lamp and is provided with a pair of electrodes that are respectively connected to the exterior via two conductors 28 and 24. Also the position of the attenuator 59 along the transmission line E4 and with respect to the position of the magnetron tube VI is such that the distance between the attenuator it and the magnetron tube VI is equal to /2 the wave length of the ultra-high frequency energy in the transmission line it or to any integral multiple of the wave length noted. This arrangement insures that the gaseous discharge tube V9 is subject to ionization by a microwave voltage at the same level of intensity as that at the magnetron tube VI.

More particularly the resulting microwave voltage to which the gaseous discharge tube V9 is subjected is proportional to the square root of the generated microwave power plus the in-phase component of that microwave voltage caused by the standing microwaves in the transmission line it, while the microwave voltage caused by the standing waves in the transmission line I4 is directly proportional to the impedance mismatch. Thus it will be understood that in the event of a resulting microwave voltage in excess of a predetermined safe value, produced by a severe impedance mismatch, the gaseous discharge tube V9 is ionized and ignited. As explained more fully hereinafter, ignition of the gaseous discharge tube V9 is effective to bring about a reduction in the microwave power that is generated by the magnetron tube VI, whereby the resulting microwave voltage to which both the gaseous discharge tube V9 and the magnetron tube VI are subjected is correspondingly reduced so that the resulting microwave voltage does not exceed the predetermined safe value in order to prevent damage to the magnetron tube VI.

The power supply system for the magnetron tube VI comprises a source of 230 volts, 3-phase, A. C. power, an associated oil circuit breaker KI that is selectively operative between open and closed positions, a plate power transformer Ti having a delta connected primary winding and a star connected secondary winding, a manually operable power selector switch SI, and a plate power relay K2. In the plate power transformer TI, the primary winding includes the three phase sections 22, 23 and Z l, and the secondary winding includes the three phase sections 25, 25 and 2! respectively coupled by an associated core 28. The power selector switch SI includes a rotatably mounted operating shaft 29 provided with an op erating handle til and carrying five switch blades 3!, 32, 33', 3t and 35 that are provided with individually associated contact banks. The plate power relay K2 includes a winding 36 and three contact bridging members 37, 38 and 39.

When the oil circuit breaker K! occupies its closed position, the three conductors of the source of power are respectively connected to three power supply buses 40, ii and 62 that are respectively terminated by contacts controlled by the bridging members 37, 38 and 38; also the other contacts respectively controlled by the bridging members 3'7, 38 and 39 respectively terminate feed conductors 43, M and 45. Thus it will be understood that when the plate power relay K2 occupies its operated position, the power supply buses 40, -H and 42 are respectively connected by the bridging members 3?, 38 and 3% to the feed conductors 43, 44 and 45.

The feed conductors 33, A l and 45 are respectively connected to the switch blades 33, 3t and 35; which switch blades 33, 3d and 35 have access to contacts in the associated contact bank re spectively terminating taps provided on the primary winding sections i l, 23 and 22. Thus it will be understood that by appropriately setting the power selector switch Si that the taps may be appropriately set upon the Winding sections 22, 23 and 24 of the primary winding of the plate power transformer Ti so as to effect the induction of corresponding secondary voltages in the secondary winding sections 25, 26 and il thereof. Finally the power supply buses it and 52 are respectively connected directly via associated fuses- FI and F2 to the switch blades 3| and 32; and the contact banks associated with the switch blades 3i and 32 are multipled together and connected respectively to two filament supply conductors Alt and tl. Thus it will be understood that when the power selector switch Si is operated out of its home position, the phase voltage between the power supply buses til and i2 is connected between the filament supply conductors t6 and il via the switch blades 3i and 32 and the engaged contacts in the associated contact banks. The outer ends of the primary winding sections 25, 2E and 2'! are respectively connected to feed connductors 43, 39 and 56; and the inner ends of the winding sections 25, 26 and 27 are commonly connected together to form the star previously noted.

Also the power supply circuit comprises a first bank of rectifier tubesN2, V3 and V4' provlded with plates respectively 'connected to the feed conductors 48, ie-and Stand provided with filamentary cathodes connected in multiple to a conductor pair :51; as well as a second bank of rectifier tubes V5, VB and V7 :provided with plates-commonly connected to an outputconductor 53-and provided with filamentary cathodes "respectively :connected to conductor pairs 54, 55 and 56; 'A rectifier. filament transformer T3 is provided *for thefirst 'bank of rectifier tubes and includes aprimary'winding 5.1 connected across the filament supply conductors :46 and 41 and :a-secondary winding I58 connected across the conductor pair iii, the mid-tap of the secondary "winding :58 being connected .toan output conductor 52. Three rectifier filament transformers T4; T and Ti; arezprovided for the respective rectifier "tubes V5, V6 and V7! in the second bank. Ifhe rectifier filament transformer T4 includes a aprima-ry Winding .59 :connected across the :filament supply conductors 46 and 4 7., and a secondary winding lid-connected across the conductor-.pair-SI, :th [mid-tap of the secondary winding "60 being connected to the iced .con'ductor #8. The 'rectifierfilament transformer T5 includes a iprimary winding :E'il connected across the filament supply conductors 146 and ll Lua-nd a secondary winding 62-connected across the conductor :pair '55, the rind-tap of the secondary winding .5.2 ibeing connected to the feed conductor 4'9. The 'rectifierrfilament transformer T6 includes a primary winding 63 connected-across the filament supply conductors 45 and 45!, and a secondary winding Muconnected across the conductor paiir 356, the mid-tap :or the secondary winding 64 beingiconnected to the-feed conductor 50.

Also the magnetron tube-Vi .is provided with a filamentpower controller Zl having input tor minals connected, across the 'filamentlsupply conductors 46. and "Al, and output terminals connec'ted a'cross aiconduc'torpairtfi. Alsothe magnetron filament .power controller Z1 is provided with an adjustable filament. controller rheostat R1 so that the voltage across theconductorpair 65 maybe selectivelylset. Also the magnetron tube Vi is provided with a filament transformer T2 including a primary winding connected across the conductor -pair .65, and .a secondary winding (i=7 connected across .thev filament Iii :of the-magnetron (tube V i, :the mid-tap :of the secondary winding 5"! being connected to the output conductor 53. Further the power supply circult comprises anisolation itransiormerTfl eluding a primary winding =.conn.ec'ted across the filament supply conductors "4.6 .=and -41 hand a secondary winding '89 connected :across two feed conductors "l0 and 'll,- Finallyithe ma'gnw tron tube Vlinlu'des a magnet :coil -LI that ;is provided 'with a local exciting circuit; the cal exciting circuit including a magnet :control transformer T8 of the auto-transformer type. Specifically the extremities of the winding &Of the transformeril fl (are connected across the feed conductorsTO and 14.; while one extremity of the winding of the transformer D8 and an adjustable tap thereon are connectedacross the input terminals ofafu'll wave dry rectifier bridge Z2; the output terminals of thebridgeTZZ being connected across a pair of conductors "l2 and 13 that are included in the excitation circuit of the magnet coil Ll. Specificall-yrone terminal "of the magnet "coil L'l is connecteddirectly by a "conductor -14 "tothe plate 18 of *the magnetron tube V1 :and to -ground potential; and the other terminalof the magnetcoilLl is connected tova conductor 15, -which,:in turn, :is connected to the:conductor L2. Theconductor n is connected via 'four resistors :Rl.l,.R1t2, :R13, :and R14 arranged in series relation to the conductor .1 4 .and

to ground potential. Further .a voltage regulator.

tube V8 of the gaseous discharge type is :connected directly across the-conductors 12-sand 13. Finally. thereis provided a direct current overcurrent :relay 1K5 that includes va'winding 116 that is connected-between the conductor 12 and the output conductor 52.

In view :of :the foregoing, it will be understood thatizthe magnet .coil L! is normally energized with :direct current =via;the local exciting circuit including-the bridge. Z2, the conductors 12, 13, T4 and Iii-and the :resistors Rlsl, R12, R13 and R! 4,.and that the highD. C. voltage producedby the two banks directiiierrtnbes :is present between the output conductors 5.2 :and 53 and thus appears between-the cathode 4:1 and .the plate ii of the magnetron duoe 'Vl via the circuit includingthe winding '16 or :the .overcurrent relay K5, theconductor'fi; ithemagnet coilLl .and the conductor :14; whereby the magnetron .tube V! oscillates .in the usual :manner impressing the generatedmicrowave voltage via the coupling loop 118a 'upon the inner conductor 16 of the transmission line :1 lso aszto-effect the-production or the centimeter electromagnetic standing Waves in the cavity t2 so as torheat the'material l3'supported therein.

The input terminals of .a voltage divider D! are hridgedacross the feed conductors 10 and l l., and the output terminals of the voltage divider DI are bridged across the input terminals of :a full wave dry rectifier bridge 23. The output terminals of the bridge Z3 are respectively connected to .tWOzCOlldllctOIs !:1 and '58. Also an ultra-high frequency overvoltage relay K5 is 'provided that includes a winding 19, one terminal of which is connected to theconductor :26. Further the input terminalsaof a voltage divider DA-are ccnnected hetween the-conductors 2.9 and 13; and the output terminals of the voltage :divider D5 are connected between the conductors 2i? and 21! that extend tothe electrodes of the glow Ilamp V9. in view'of the foregoing, it will be understood that when the glow slamp V9 is LlIl-lOIIiZfid that'the winding 19 =of.-the ultra-high irequencyovervoltage"relay K6 .is energized in series .-relation with the voltag divider J34, whereby the latter relay is not operated since it is of the marginaltype. On the other hand, when the;glowtlamp v.fi is ionized, theup-per portion of the VOitflfIErdiVidEYD4llS efiectively shortcircuited, whereby the Winding .19 of the ultrahigh frequency'overvoltage relay 1. .6 isenergized sufiiciently to effect operation thereof. Subsequently when the glow lamp V9 is :again deionized, the upper portion of the voltage divider D! is againeffectively-inserted in the series circuit rfor-energizingthe Winding ;'!.3 of the ultrahigh frequency overvoltage relay :K5 'in order to cause the latter relay to restore.

'Further' the 'circuit network comprises a. con.- trol switch S-S of thestep-by-step type including an operating shaft .8 0 mounted for rotation stepby-step andcarryingawiper 8! having a home position illustrated, a plurality of control positions-and a ifinal-iposition disposed in the clock- Wise direction with respect to its home position. The wiper;-Bl :is-lconn'ected to the conductor 1.3 andis provided with a-contactzbank terminating conductors extending to the junctions between the resistors RI I, Ri2, Hi3 and RM and the conductor 14. Thus it will be understood that the resistors RH, R12, RIS and RIG are normally included in the local exciting circuit for the magnet coil Li and that the resistors Bi 1, R12, Rlii and RM are successively short-circuited by the wiper 81 as the control switch SS is stepped from its home position into its successive control positions, thereby successively removing the resistors RI l, RiZ, R13 and RM from the exciting circuit for the magnet coil Li. The magnetron tube VI of the type Z-1492 possesses the characteristic that the ultra-high frequency power output thereof is inversely proportional to the excitation of the magnet coil Ll thereof. Thus the ultrahigh frequency power output of the magnetron tube VI is at a normal maximum when the four resistors Ri I, Hi2, RH and RM are included in the local exciting circuit for the magnet coil Ll. Also the operating shaft 811 carries a wiper 82 that is adapted to short together the conductors 2G and 2! when the control switch SS occupies its final position, thereby short-circuiting the glow lamp V9 and the upper portion of the voltage divider D4 so as to provide a locking circuit positively to retain the ultra-high frequency overvoltage relay K6 in its operated position.

The control switch SS further comprises a coil spring 83 normally biasing the operating shaft 80 in the counterclockwise direction; which coil spring 83 is wound up, storing energy therein, when the operating shaft 80 is stepped away from its home position. The control switch SS is stepped away from its home position by an arrangement including a stepping magnet SM provided with an armature 84 that carries an operating pawl 85 that cooperates with a ratchet wheel 86 carried by the operating shaft 88, the armature 84 being biased in the clockwise direction away from the stepping magnet SM by a coil spring ill. The operating shaft 8!] is restrained in its operated position against the bias of the coil spring 83 by a latching pawl 88 that is carried by an armature E9 in cooperating relation with respect to the ratchet wheel 86. The armature 89 is operatively associated with a release magnet RM and is normally biased in the clockwise direction away from the release magnet RM by a coil spring 90.

In view of the foregoing, it will be understood that the control switch SS normally occupies its home position; however when a series of impulses are received by the stepping magnet 'SM, the armature at is alternately attracted and released so that the stepping pawl 85 cooperating with the ratchet wheel 86 rotates the operating shaft 80 step-by-step in the clockwise direction so that the wiper 8! is rotated out of its home position through its control positions and ultimately into its final position. The operating shaft all is restrained in any operated position by the latching pawl 88; however the operating shaft 80 may be released and returned in the counterclockwise direction by the coil spring 83 so that the wiper 8! is moved back into its home position merely by the momentary energization of the release magnet RM, the release magnet RM attracting the armature 89 and withdrawing the release pawl 88 from the ratchet wheel 86.

For the purpose of energizing the stepping magnet SM a circuit is provided that includes a voltage divider D2 having input terminals connected across the feed conductors l and l i, and output terminals connected across the input terminals of a full wave dry rectifier bridge Z4. The output terminals of the bridge Z4 are connected to two conductors 9| and 92. The conductor Si is connected to one terminal of the stepping magnet SM, and the other terminal of the stepping magnet SM is connected to a contact controlled by a bridging member 93 provided on the ultra-high frequency overvoltage relay K6, the other contact controlled by the bridging member 93 being connected to the conductor 92. Thus it will be understood that when the ultra-high frequency overvoltage relay K6 is operated the bridging member 93 closes the circuit for energizing the stepping magnet SM of the control switch. Also the power supply circuit comprises a plate power delay relay K3 that may take the form of a Telechron timer motor having an operating winding connected across the filament supply conductors Z6 and 41 and provided with an operating shaft 9d. Thus the plate power delay relay K3 is of the slow-tooperate type, the shaft 94 thereof being operated 30 seconds after the winding thereof is energized. For the purpose of energizing the release magnet RM, a circuit is provided that includes a voltage divider D3 having input terminals connected across the feed conductors i3 and ll via a pair of contacts controlled by a bridging member 96 provided on the plate power delay relay K3, and output terminals connected across the input terminals of a full wave dry rectifier bridge Z5. The output terminals of the bridge Z5 are connected directly to the terminals of the release magnet RM. Thus it will be understood that when the plate power delay relay K3 occupies its restored position that the circuit for operating the release magnet RM is closed.

Further in the power supply circuit arrangement a water coil, not shown, is operatively associated with the magnetron tube V! and is supplied with cooling water during operation of the magnetron tube VI. In this water cooling arrangement, a water flow interlock switch S3 is provided that occupies its closed position only when the water is flowing through the water cooling coil noted; and a water temperature interlock switch S2 is provided that occupies its closed position only when the temperature of the water after it has passed through the water cooling coil is at a safe operating temperature. Further the oven I0 is provided with a door, not shown, with which an oven door interlock switch S4 is operatively associated, the oven door interlock switch S4 being closed only when the oven door noted occupies its closed position. Finally a timer K4 is provided that includes an electric drive motor 9! of the synchronous type, and preferably of the Telechron type, a gear box 98, a slip clutch 99, a control cam I00, and a manually operable control dial [0]. Specifically the drive motor 91 is provided with a drive shaft Hi2 that terminates in the gear box 98; and the gear box 98 is provided with a driven shaft I03 that terminates in one of the clutch plates of the slip clutch 99; and the control cam I99 and the control dial lill are rigidly secured to an operating shaft EM terminating in the other clutch plate of the slip clutch 99. Thus the operating shaft HM may be rotated by the control dial Hli independently of the gear box 98 by virtue of the arrangemnt of the slip clutch 99, whereby the control earn it!) is rotated therewith eifecting closure of an associated pair of switch springs I05. Operation of the drive motor 91 returns the operating shaft Hi l back toward its home position at a timed rate, whereby the control cam I is returned back into its normal position at the expiration of the set time in order: again to: open the set of switch springs I65. Accordingly it will be understood that when the control dial I'Ol is rotated. out of its normal position to set a variable time that the drive motor 91 returns the operating shaft [.04 back into its normal position after the expiration of. the preset time, whereby the control cam I00. effects closure of the. set of switch springs I during the preset time noted.

The winding 36 of the plate ower relay K2 is provided with an operating circuit that includes the feed conductors and 11, the set of. switch springs 105, the water temperature interlock switch S2, the. water flow interlock switch S3, the oven door interlock switch $4, a pair'ofv contacts controlled by a bridging member I06 provided on the plate power delay relay K3, a pair of. contacts controlled by a bridging. member I 01 provided on. the direct current overcurrent relay K5,. and apair of contacts. controlled by a bridging memher 168 provided on the ultra-high frequency overvoltage relay K5. Further a water pump, not

shown, is provided for the purpose of circulating water throughthe cooling coil, not shown, of the magnetron tube VI, the water pump being operatedbyawater pump motor BI connected betweenithefilamentsupply conductors 46 and 41. The water: mentioned is circulated through a cooler, not shown, that is providedwith. a cooling fan, not shown, that is operated by a water cooling fan motor B2 that is connected across the filament supply conductors 46 and 41. The magnetron Lube VI is also provided with a cooling blower, not shown, that is operated by a mag.- netrou cooling blower motor B4 that is connected across the filament supply conductors 46 and 41. The rectifier tubes V2, etc. are providedwith a rectifier cooling blower, not shown, that is operated by a rectifier cooling blower motor B3 that is connected across the feed conductors 10 and 1|.

Considering now the general operation ofv the power supply circuit, the power selector switch S1 is first manually set into one of its positions corresponding to the desired ultra-high frequency power level output of the magnetron tube VI the material l3 to be heatedis placedin the cavity [2; the door of the oven II]. is closed in order to effect closure of the oven door interlock switch S4; the manual dial ll?! is rotated out of its normal position-to setv a time during which it is desired to heat the material I 3 in the cavity l2 so as to effect closure of the set of switch springs [65; and finally the oil circuit breaker Kl is operated into its closed position. Closure of the oil circuit breaker Kl effects thesupply of power tothe filament supply conductors 46 and 41', whereby the primary winding 68 of the isolation transformer T1 is energized to effect the supply of power to the feed conductors 10 and H. The power supplied to thefilament supply conductors 46 and 41 effects operation of the water pump .motor BI, the water cooling fan motor B2, and the magnetron cooling blower motor B4; while thesupply of power to the feedconductors 10 and 11 effects operation of the rectifier cooling blower motor B3. Also the supply of power to the filament supply conductors 46 and.41 energizes the primary windings 51, 59., Ii'l and 63 of the rectifier filament transformers T3, T4, T5 and T6, .whereby voltages are inducedin the respective secondary windings 58, 60, 6'2 and 64, thereof, so as to cause heating of the filamentary cathodes of the rectifier tubes V2, V3, V4, V5, V6 and V1. Also the supply of power to the filamentary supplyiconductors 46 and 41. efiects the energization ot'the primary winding [iii of themagnetron filamentv transformer T2 through the associated magnetronifilament power'controller Zt, the associated filament control rheostat RA having been previously set to establish the desired filament supply current for'the' filamentary cathode l"; of themagnetron tube Vi, whereby thevoltage inducedsinl-the secondary winding 61; of the magnetron. filament transformer T2 eiecte heating of the cathode. I1 (tithe-magnetron tube VI.

Further the supply of power to the feed condoctors 19 and; 1,1 closes the previously-traced local: exciting,"oircuitincluding the rectifier bridge Z2 andtheresistorsR'lI,.R;| 2, Hi3. and RM for the magnet coil-Lt of the magnetron. tube Vl, whereby the ultra-high frequency power of the magnetron tube Vl. is preset at the maximum value: Also the-:supply' of power: to the feed conductors 1lli and-.11 completes the previously-traced circuit including therectifier bridge Z5 for operating, the release: magnet RM of the control switch SS soaszt'oz-insure that the control switch SS occupies its, normal or home position at this time; the lastementioned circuit being completed by'virtue of the restored position of the plate power idelay relay K3 at this time. Further the supply of powerxto the feed conductors 10 and 1| completes the previously-traced circuit including the rectifier bridge-Z3. andthendivider D4 for energizing, the winding: 18 or the ultra-high frequency overvolt'age relay K6, whereby the latter relaybeingzofithe. marginal type remainsinits restored; positi'on'at this-time Finally the supply of powerto: the filamentsupplyconductors Miami 41 effectetheenergizationof the winding of the plate power delay relay K3, whereby thelatter relay beingof the slow-to-operate type is operated only after a predetermined timeintervaLabout 30 secon'ds; so as to permit l heating; of the filamentary cathodes of. the various rectifier tubes V2, etc., andheating' of the; filamentary cathode I 1 of the magnetron tube V l Upon operatingthe platepower delay relay K3 interrupts. at therbridging member 96, the previously-traced circuit for operating the release magnet RM of the control switch SS so as to efiect the'releaserofthe. armature, whereby the control switch SS-is-oonditioned to be operated away fromlits;homeposition. Also upon operating the plate power delay. relay K3wcompletes, at the bridging member 166-, the previously mentioned seriesicircuit' for energizing. the winding 36 of the plate. power relay K2, this circuit including the feed. conductors 1 0 and1 I, the bridging members [266, I61 and" 1.08,:thewater temperature interlock switch; S2; the water flow interlock switch S3, the oven door interlock switch S4, and the set of switchl springs [0510f the: timer K4. When this circuit is completed, the plate power relay K2 operates. connecting the power supply buses 48, 4| andl42'tolthefeedconductors '43, and 45 Whereby-the primary winding: sections 22, 23 and 24 of theplate power transformer TI. is. energized. so that'poweris-supplied-viai the feed conductors 48, 49 and 50 totherectifier tubes V2, V3, V4, V5, V6 and V1. Also the drive 'motor'9 1- of the timer K4 is energized in multiple with the winding 36 of the plate powersupply relay K2-so that the operating shaft I64 is rotated back toward its home position through the gearbox 98 and the associated slip clutch 99 so that timin of operation of the supply circuit is initiated by the timer K4. Also'it is noted that a pilot lamp I2 is illuminated in multiple with the winding 36 of the plate power relay K2 in order to indicate that operation of the timer K4 has been initiated and that the plate power relay K2 occupies its operated position. Further a pilot lamp II is illuminated directly across the feed conductors Ill and ll in order to indicate the supply of power thereto from the isolation transformer T1.

The rectifier tubes V2, etc., effect the supply of high voltage direct current to the output conductors 52 and 53 in a well-known manner so that oscillation of the magnetron tube VI is initiated, whereby the ultra-high frequency power is supplied from the magnetron tube Vi via the transmission line I4 to the cavity I2 eifecting heating of the material I3 therein. Under standard and normal conditions, operation of the power circuit proceeds until the expiration of the preset time interval, whereupon the control cam I90 is returned back into its normal position effecting opening of the set of switch springs Hi5 and the consequent de-energization of the winding 36 of the plate power relay K2, extinguishing of the pilot lamp I2, and arresting of operation of the drive motor 91. The plate power relay K2 then restores disconnecting at the bridging members 36, 31 and 38 the power supply buses III, 4| and d2 from the feed conductors 43, 44 and 65 so as to eifect de-energization of the plate power transformer T! in order to arrest operation of the rectifier tubes V2, etc. and the magnetron tube VI. When the pilot lamp I2 is thus extinguished it indicates that it is safe to open the door, not shown, of the oven I so that the material I3 may be removed from the cavity I2. When the door,

not shown, of the oven III is thus opened, the

oven door interlock switch S4 is returned into its open position.

During operation of the power supply circuit, should the flow of water through the water cooling coil, not shown, associated with the magnetron tube VI be arrested, the water flow interlock switch S3 would be returned into its open position effecting the restoration of the plate power relay K2. In a similar manner, should the temperature of the water flowing through the water cooling coil, not shown, associated with the magnetron tube VI exceed a safe temperature, the water temperature interlock switch S2 would be operated into its open position efiecting the restoration of the plate power supply relay K2. Further should the door, not shown, of the oven Ill, be inadvertently opened, the oven door interlock switch S4 is returned into its open position effecting the restoration of the plate power relay K2. Also should the control dial IUI of the timer K4 be prematurely returned manually back into its normal position the set of switch springs I05 is opened effectingthe restoration of the plate power relay K2. Moreover during the operation of the power circuit should the magnetron tube V! draw an abnormally high overload current, the direct current overcurrent relay K5 is operated in order to interupt, at the bridging member I01, the circuit for energizing the winding 36 of the plate power relay K2 so as to effect the restoration thereof.

Reconsidering the operation of the power circuit, the impedance of the load comprising the material I3 disposed in the cavity I2 normally reasonably matches the impedance of the magnetron tube VI, whereby there is no excessive reflected ultra-high frequency power from the cavity I2 back to the magnetron tube VI. However in the event of a substantial mismatch between the impedance of the load in the cavity I2 and the impedance of the magnetron tube VI, there is a substantial reflected ultra-high frequency power from the cavity I2 back to the magnetron tube V whereby an excess and unsafe microwave voltage is present across the magnetron tube VI that would effect undue and damaging heating thereof. However this excess microwave voltage greater than a safe value, due to the reflected ultra-high frequency power, is present in the transmission line M, and consequently a portion thereof is present in the attenuator I9 so that the glow lamp V 9 is ionized and ignited. When the glow lamp V9 is thus ignited, the upper portion of the divider D4 is effectively short-circuited eifecting sufiicient energization of the winding I9 of the ultra-high frequency overvoltage relay Kt to cause operation thereof.

Upon operating the ultra-high frequency overvoltage relay K6 interrupts, at the bridging member I88, the circuit for energizing the winding 36 of the plate power relay K2 so as to effect restoration of the latter relay and the consequent removal of power from the plate power transformers TI and the consequent protection of the magnetron tube VI. Also the ultra-high frequency overvoltage relay K6 completes, at the bridging member 93, the previously-traced circuit for energizing the stepping magnet SM of the control switch SS, whereby the armature M is attracted causing the pawl 85 cooperating with the ratchet wheel 65 to drive the operating shaft 853 one step in the clockwise direction out of its home position into its first control position so that the wiper BI short-circuits the resistor RII normally included in the local exciting circuit for the magnet coil Ll. The operating shaft of the control switch SS is restrained'in its first control position by the cooperation between the release pawl 38 and the ratchet wheel I36, and energy is stored in the coil spring 83.

When operation of the magnetron tube VI is arrested, no incident ultra-high frequency power is supplied via the transmission line I l to the cavity I2, whereby the glow lamp V9 is extinguished again inserting the upper portion of the divider D4 into series relation with the winding I9 of the ultra-high frequency overvoltage relay K6 so as to cause the latter relay to restore since it is of the marginal type. Upon restoring the ultra-high frequency overvoltage relay K6 interrupts, at the bridging member 93, the circuit for operating the stepping magnet SM of the control switch S5 so that the spring 81 may return the armature 84 into its normal position thereby to condition the control switch SS to be operated another step in the event of reoperation of the stepping magnet SM. Also upon restoring the ultra-high frequency overvoltage relay K5 recompletes, at the bridging member I538 the circuit for energizing the winding 36 of the plate power relay K2, whereby the latter relay reoperates in order again to effect the supply of power to the plate power transformer TI so that the respective tubes V2, etc., and the magnetron tube V I are again operated in order to supply the ultra-high frequency power from the magnetron tube VI via the transmission line It to the cavity In view of the foregoing, it will be understood that at this time the incident ultra-high frequency power that is supplied from the magnetron tube VI via the transmission line 5 to the cavity I2 is at a given lower predetermined level by virtue of the circumstance that the control switch SS occupies its first control position shortcircuiting the resistor RII. so that the magnet coil LI is excited with a greater current in its local exciting circuit. Also it will beappreciated that in the event the impedance mismatch is sufiiciently severe, again an undue and dangerous amount of ultra-high frequency power willl be reflected back from the cavity I2 through the transmission line I4 to the magnetron tube VI so as again to ignite the glow tube V9, whereby the circuit is recycled in the manner explained above. Specifically the ultra-high frequency overvoltage relay K8 is operated and then restored effecting restoration and then reoperation of the plate power relay K2 and driving of the control switch SS from its first control po sition into its second control position. Thus at this time incident ultra-high frequency power is supplied from the magnetron tube VI via the transmission line M to the cavity I2 at a still lower given level by virtue of the exclusion of the: resistors RI 1 and RIZ from the local exciting circuit of the magnet coil LI of the magnetron tube-VI.

In view of the foregoing description .of the mode of operation of the glow lamp V9 and the associated ultra-high frequency overvoltage relay K6, in conjunction with the plate power relay K2 and control switch SS, it will be understood that successive operations of the control switch SS are effected until the incident ultra-high frequency power level of the magnetron tube .VI: is established, in view of the impedance mismatch, so that the amount of reflected ultra-high frequency power from the cavity I2 via the trans mission line I4 to the magnetron tube VI is not excessive. and at a safe level so that damaging heating of the magnetron tube VI is prevented.

Further it is'pointed out that in the event of an extremely severe impedance mismatch that the glow lamp V9 is successively ignited so that the control switch SS is driven through its various control positions into its final position wherein the wiper 82 engages the contact in the associated contact bank completing a direct connection between the conductors and H, thereby short-circuiting the upper portion of the divider D4 in multiple with the glow lamp V9 and effecting the completion of a locking circuit for energizing the winding 19 of the ultra-high frequencyovervoltage relay K6 so as to retain the latter relay in its operated position notwithstanding the subsequent extinguishing of the glow lamp V9 when the plate power relay K2 restores. Thus in this event the ultra-high frequency overvoltage relay K6 is locked in its operated position in order to prevent further reoperation of the plate power relay K2 so that the power circuit is locked out of service. In this connection, it is noted that the ultra-high frequency overvoltage relay K6 also includes a contact bridging member I09 that completes a circuit for illuminating an alarm lamp AL when the latter relay occupies its operated position; the circuit mentioned being bridged across the conductors II and 78. The continued illumination of the alarm lamp AL indicates that the control switch SS has been operated into its final position and that the power circuit has been locked out of service. The attendant then trips the oil circuit breaker KI into its open position in order to remove power from the power supply buses 40, 4| and 42 bringing about the restoration of the plate power delay relay K3 and the ultra-high frequency overvoltage relay KB.

The attendant may again place the power supshaft of the control switch SS is thus returned into its home position, the resistors RI I, etc-., are again inserted in the local exciting circuit for the magnet coil LI of the magnetron tube VI. Also when the oil circuit breaker K! is reclosed, the winding of the plate power delay relay K3 is energized so as to effect operation of the latter relay after the time interval previously noted. Also upon reoperating the plate power delay relay K3' interrupts, at the bridging member 96, the circuit for energizing the release magnet RM of the control switch SS and effects reoperation of the plate power relay K2 so that the magnetron tube VI is reoperated. Of course in the event the severe impedance mismatch has not been corrected, the glow lamp V9 will be again successively ignited to eifect successive recycling of the ultrahigh frequency overvoltage relay K6 and the ultimate locking of the latter relay in its operated position and the continued illumination of the alarm lamp AL, in the manner explained above.

For purposes of instrumentation a. plate voltmeter MI is bridged between the output conductor 53 and ground potential; and a plate ammeter M2 is included in the output conductor 52. Also a magnet current ammeter M3 is included in the conductor I5 in series relation with the magnet coil LI; a filament voltmeter M6 is bridged across the conductor pair 65 for the purpose of measuring the voltage supplied to the primary winding 66 of the magnetron filament transformer T2; a filament hourmeter M7 is bridged across the filament supply conductors 46 and 41' and may comprise essentially a synchronous motor of the Telechron type; and a plate hourmeter M8 is bridged across the feed conductors 44 and 45 and may comprise essentially a synchronous motor of the Telechron type. Alsoan incident ultra-high frequency power wattmeter M4 and a reflected ultrahigh frequency power wattmeter M5 are provided. More particularly two directional couplers I5 9 and III are respectively connected between the outer conductor I5 of the transmission line I4 and two conductors H2 and H3, which conductors H2 and H3 are respectively connected to two square law crystal detectors I I4 and I I5 that may be of the germanium crystal type. In turn, the detectors I I4 and I I5 are connected to a filter network that comprises two resistors I 56 and I I7 and two smoothing condensers I I8 and I I9, the midpoint between the resistors H6 and II! and the midpoint between the smoothing condensers II8 and [I9 being commonly connected together by a conductor I20, that is, in turn, connected both to ground potential and to the outer conductor I5 of the transmission line I I. The incident ultra-high frequency power wattmeter M4 is bridged between the outer extremities of the resistor H6 and the condenser I I8 and the conductor I28; and the reflected ultra-high frequency power wattmeter M5 is bridged between the outer extremities of the resistor II! and the condenser [I9 and the conductor I20. In the operation of thev meters M4 and M5, it is pointed out that the directional coupler I I0 is arranged and poled with respect to the incident ultra-high frequency elec- 'tromagnetic traveling wave in the transmission line it to derive a first ultra-high frequency current that is proportional to the incident voltage of the wave noted; while the directional coupler H! is arranged and poled with respect to the reflected ultra-high frequency electromagnetic traveling wave in the transmission line it to derive a second ultra-high frequency current that is proportional to the reflected voltage of the wave noted. The first current mentioned derived by the directional coupler H is rectiiied by the square-law crystal detector 1 M to produce a third ultra-high frequency puisating current that is proportional to the square of the first current mentioned; and the second current mentioned derived by the direc tional coupler H! is rectified by the square-law crystal detector I E5 to produce a fourth ultra-high frequency pulsating current that is proportional to the square of the second current mentioned. The third and fourth currents mentioned are applied in opposition to each other to the extremities of the resistors HE and Ill, whereby the smoothing condensers l l8 and 1 i9 produce unidirectional voltages that are respectively proportional to the incident ultra-high frequency power and to the reflected ultra-high frequency power, which voltages are respectively applied to the meters M4 and M5 that may be of the voltmeter type. Accordingly the meters M4 and M5 effectively measure the respective incident ultra-high frequency power and the reflected ultra-high frequency power in the transmission line it.

The construction and arrangement of the cooking oven l0 and the associated transmission line 54 provided with the bifurcated end terminating in the two arms embracing the oven Ill are disclosed and claimed in the copending application of George W. Schroeder, Serial No. 232,005, filed June 16, 1951.

In view of the foregoing, it is apparent that there has been provided an improved protective and control circuit network for an ultra-high frequency power supply system so as to prevent damage to the ultra-high frequency generator in the event of an impedance mismatch between the load and the generator and so as to establish automatically a safe ultra-high frequency power level of operation of the ultra-high frequency generator in view of the existing impedance mismatch between the generator and the load.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a radio frequency power supply system including a load, a radio frequency generator of the magnetron type provided with an input circuit and an output circuit and a magnet coil, a source of power, a power switch selectively operative to connect and to disconnect said source of power with respect to said input circuit, means for connecting said output circuit to said load, and an exciting circuit for said magnet coil, the radio frequency power supplied by said magnetron to said load via said output circuit being inversely proportional to the excitation of said magnet coil via said exciting circuit; the combination comprising means for setting a predetermined excitation of said magnet coil via said exciting circuit, means for closing said power switch, means including an element for testing the amount of radio frequency power reflected from said load back to said mag netron via said output circuit, a relay, means controlled by said element in response to testing of a certain amount of radio frequency power reflected from said load back to said magnetron via said output circuit for operating said relay, means controlled by operation of said relay for opening said power switch and for setting a higher given excitation of said magnet coil via said exciting circuit, means controlled by interruption of the supply of radio frequency power from said magnetron to said lead via, said output circuit for restoring said relay, and means controlled by restoration of said relay for reclosing said power switch.

2. In a radio frequency power supply system including a load, a radio frequency generator of the magnetron type provided with an input circuit and an output circuit and a magnet coil, a source of power, a power switch selectively operative to connect and to disconnect said source of power with respect to said input circuit, means for connecting said output circuit to said load, and an exciting circuit for said magnet coil, the radio frequency power supplied by said magnetron to said load via said output circuit being inversely proportional to the excitation of said magnet coil via said exciting circuit; the combination comprising a device successively operative from a normal position through a plurality of control positions into a final position, means controlled by said device in its normal position for setting a predetermined excitation of said magnet coil via said exciting circuit and controlled by said device in its successive control positions for setting successively higher given excitations of said magnet coil via said exciting circuit, means for closing said power switch, means for operating said device into its normal position, an element operative in response to the reflection of radio frequency power from said load back to said generator in a certain amount, a relay, means controlled by operation of said element for operating said relay, means controlled by operation of said relay for opening said power switch and for operating said device from its present position into the adjacent one of its positions and toward its final position, means normally controlled by interruption of the supply of radio frequency power from said magnetron to said load via said output circuit for restoring said relay, means controlled by restoration of said relay for reclosing said power switch so that said testing means again tests the amount of radio frequency power reflected from said load back to said magnetron via said output circuit, and means controlled by operation of said device into its final position for retaining operated said relay.

3. The combination set forth in claim 2, and further comprising release means for restoring said relay and for operating said device from its final .position back into its normal position.

i. The combination set forth in claim 2, wherein said exciting circuit includes a plurality of series related resistors, and said device is in the form of a stepping switch arranged successively to remove said resistors from said exciting circuit in the successive control positions thereof.

5. In a radio frequency power supply system including a load, and a radio frequency generator operative to supply radio frequency power to said load; the combination comprising a device successively operative from a normal position through a plurality of control positions, means controlled by said device in its normal position for setting said generator to supply radio frequency power in a predetermined amount to said load and controlled by said device in its successive control positions for setting successively said generator to supply radio frequency power in successively lower given amounts to said load, means for operating said device into its normal position, means for operating said generator, means including an element for testing successively the amount of radio frequency power reflected from said lead back to said generator, and means controlled by said element in response to each testing of a certain amount of radio frequency power reflected: from said load back to said generator for operating said device away from its normal position.

6. The radio frequency power supply system combination set forth in claim 5, wherein said generator essentially comprises a magnetron type tube.

7; The radio frequency power supply system combination setforth in claim 5, wherein said element essentially comprises a gaseous discharge type tube.

8. The radio frequency power supply system combination set forth in claim 5, wherein said device essentially comprises a switch of the stepby-step type.

9. In a radio frequency power supply system including a load, and a radio frequency generator operative to supply radio frequency power to said load; the combination comprising a device successively operative from a normal position through a plurality of control position into a final position, means controlled by said device in its normal position for setting said generator to supply radio frequency power in a predetermined amount to said load and controlled by said device in its successive control positions for setting successively said generator to supply radio frequency power in successively lower given amounts to said load, means for operating said device into its normal position, means for initiat ing operation of said generator, means including an element for testing successively the amount of radio frequency power reflected from said load back to said generator, means controlled by said element in response to each testing of a certain amount of radio frequency power reflected from said load back to said generator for operating said device away from its normal position, and means controlled by operation of said device into its final position for arresting operation of said generator.

10. In a radio frequency power supply system including a load, a radio frequency generator operative to supply radio frequency power to said load, a source of power, and a power switch operable between open and closed positions, said power switch in its closed position connecting said source of power to said generator in order to effect operation thereof; the combination comprising a device successively operative from a normal position through a plurality of control positions into a final position, means controlled by said device in its normal position for setting said generator to supply radio frequency power in a predetermined amount to said load and controlled by said device in its successive control positions for setting successively said generator to supply radio frequency power in successively lower given amounts to said load, means for operating said power switch into its closed position, means for operating said device into its normal position, mean including an element for testing successively the amount of radio frequency power reflected from said load back to said generator, means controlled by said element in response to each testing of a certain amount of radio frequency power reflected from said load back to said generator for operating said device away from its normal position, means controlled by operation of said device into its final position for operating said power switch into its open position, and additional means responsive to the supply of more than said predetermined amount of radiofrequency power from said generator to said load for operating said power switch into its open position.

11. In a radio frequency power supply system inciuding a source of power, a supply bus, a first switch operable between open and closed positions, said first switch in its closed position connecting said source of power to said supply bus, a load, a radio frequency generator operative to supply power to said lead, and a second switch operable between open and closed positions, said second switch in its closed position connecting said supply bus to said generator in order to effect operation thereof; the combination comprising a first relay of the slow-to-operate type, means for operating said first switch into its closed position, means controlled by the supply of power to said supply bus for initiating operation of said first relay, means controlled by operation of said first relay for operating said second switch into its closed position, a device successively operative from a normal position through a plurality of control positions into a final position, means controlled by said device in its normal position for setting said generator to supply radio frequency power in a predetermined amount to said load and controlled by said device in its successive control positions for setting successively said generator to supply radio frequency power in successively lower given amounts to said load, means for operating said device into its normal position, means including an element for testing successively the amount of radio frequency power reflected from said load back to said generator, a second relay, means controlled by said element in response to each testing of a certain amount of radio frequency power reflected from said load back to said generator for operating said second relay, means controlled by operation of said second relay for operating said second switch into its open position and for operating said device away from its normal position, means normally responsive to the disconnection of said supply bus from said generator for restoring said second relay, means responsive to restoration of said second relay for reoperating said second switch into its closed position, and means controlled by operation of said device into its final position for retaining operated said second relay.

12. In a radio frequency power supply system including a source of power, a supply bus, a first switch operable between open and closed positions, said first switch in its closed position connecting said source of power to said supply bus, a load, a radio frequency generator operative to supply power to said load, and a second switch operable between open and closed positions, said second switch in its closed position connecting said supply bus to said generator in order to effect operation thereof; the combination comprising a first relay of the slow-to-operate type, means for operating said first switch into its closed position, means controlled by the supply of power to said 1i) supply bus for initiating operation of said first relay, means controlled by operation of said first relay for operating said second switch into its closed position, a device successively operative from a normal position through a plurality of control positions into a final position, means controlled by said device in its normal position for setting said generator to supply radio frequency power in a predetermined amount to said load and controlled by said device in its successive control positions for setting successively said generator to supply radio frequency power in successively lower given amounts to said load, means including an element for testing successively the amount of radio frequency power reflected from said load back to said generator, a second relay, means controlled by said element in response to each testing of a certain amount of radio frequency power reflected from said load back to said generator for operating said second relay, means controlled by operation of said second relay for operating said second switch into its open position and for operating said device away from its normal position, means normally responsive to the disconnection of said supply bus from said generator for restoring said second relay, means responsive to restoration of said second relay for reoperating said second switch into its closed position, means controlled by operation of said device into its final position for retaining operated said second relay, means for operating said first switch into its open position, means responsive to the removal of power from said supply bus for restoring said first and second relays, and means controlled by the restoration of said first relay for operating said device from its final position back into its normal position.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,374,008 Godsey Apr. 1'7, 1945 2,498,719 Spencer Feb. 28, 1950 2,498,720 Wild Feb. 28, 1950 2,508,548 Spauldine May 23, 1950 2,546,500 Hall Mar. 27, 1951 2,595,748 Andrews May 6, 1952 2,648,772 Dawson et a1. Aug. 11, 1953 

